It was posted here in 2007, just before I completed the Picaxe based remote start/monitoring controller. Alas, you can't see the roller as it's facing the wall. It's about 2.25" diameter, 2" wide. That's a small oiler in the air line right at the motor. The Gast 4AM motor does a good starting job on the 6/1 Listeroid even with unloaded air compressor but must have decompression. The 12/2 is likely too big for it. Someone tried a 6AM motor on a 12/2 and then switched to electric- the 6AM is a massive air eater, and plumbing for that much airflow from a tank will be a challenge.
Pressure loss in the line significantly affects even the 4AM motor, which must have 1/2 hose supply, high flow solenoid, and short 1/2 exhaust to a 3/4 pipe tractor muffler (low back pressure). The motor only sees the difference between input and exhaust pressures...a real eye opener when gauges are put on each next to the motor inlet/outlet under power.
I'm not familiar with your Gast serial number. Their standard product is in various family sizes, 1AM, 3AM, 4AM, 6AM. They also made contract units of oddball models. I use the 1, 3 and 4 AMs, I don't have the air supply for the 6AM.
McMaster.com has rubber rollers. I used one with a 1 inch bore that fit a 1" OD shaft coupler and a fairly soft rubber for good flywheel grip. Still using the original roller 12 years and 3K engine hours later. A few starts daily for 20 minute runs when I'm working in the shop, for filling the 500 gallon air tank. If I forget it shuts itself down after 10 minutes of no power or air load. A pressure transducer and circuit can see the pulses of air pumping...and when the air unloader pilot valve has turned it off.